From c0ff3d92ad732d5de1a63bfb6627a37edb672889 Mon Sep 17 00:00:00 2001
From: Rangi <sylvie.oukaour+rangi42@gmail.com>
Date: Mon, 13 Jul 2026 22:38:57 -0400
Subject: [PATCH] Fix reverse palette offsets when base palette ID wraps around
 to 0

---
 src/gfx/reverse.cpp                |  19 +++++++------------
 test/gfx/reverse_base_wrap.2bpp    |   1 +
 test/gfx/reverse_base_wrap.flags   |   5 +++++
 test/gfx/reverse_base_wrap.pal     | Bin 0 -> 64 bytes
 test/gfx/reverse_base_wrap.palmap  |   2 ++
 test/gfx/reverse_base_wrap.tilemap | Bin 0 -> 8 bytes
 6 files changed, 15 insertions(+), 12 deletions(-)
 create mode 100644 test/gfx/reverse_base_wrap.2bpp
 create mode 100644 test/gfx/reverse_base_wrap.flags
 create mode 100644 test/gfx/reverse_base_wrap.pal
 create mode 100644 test/gfx/reverse_base_wrap.palmap
 create mode 100644 test/gfx/reverse_base_wrap.tilemap

diff --git a/src/gfx/reverse.cpp b/src/gfx/reverse.cpp
index 897a4690..13ee60b5 100644
--- a/src/gfx/reverse.cpp
+++ b/src/gfx/reverse.cpp
@@ -293,19 +293,13 @@ void reverse() {
 		// 1. Checking those during the main loop is harmful to optimization, and
 		// 2. It clutters the code more, and it's not in great shape to begin with
 		for (size_t index = 0; index < mapSize; ++index) {
-			uint8_t attr = (*attrmap)[index];
 			size_t tx = index % width, ty = index / width;
+			uint8_t attr = (*attrmap)[index];
+			uint8_t palID = attr & 0b111;
 
-			if (uint8_t palID = attr & 0b111; palID < options.basePalID) {
-				error(
-				    "Attribute map references palette #%" PRIu8
-				    " at (%zu, %zu), but the base palette ID is #%" PRIu8,
-				    palID,
-				    tx,
-				    ty,
-				    options.basePalID
-				);
-			} else if (palID >= palettes.size() + options.basePalID) {
+			// The unsigned underflow for `palOfs` is intentional, since a nonzero
+			// base palette ID may overflow and continue with IDs from 0.
+			if (uint8_t palOfs = (palID - options.basePalID) & 0b111; palOfs >= palettes.size()) {
 				error(
 				    "Attribute map references palette #%" PRIu8
 				    " at (%zu, %zu), but there are only %zu palette%s",
@@ -535,7 +529,8 @@ void reverse() {
 			            : index;
 			// This should have been enforced by the earlier checking.
 			assume(tileOfs < nbTiles + options.trim);
-			size_t palOfs = (palmap ? (*palmap)[index] : attribute & 0b111) - options.basePalID;
+			size_t palOfs =
+			    ((palmap ? (*palmap)[index] : attribute & 0b111) - options.basePalID) & 0b111;
 			assume(palOfs < palettes.size()); // Should be ensured on data read
 
 			// We do not have data for tiles trimmed with `-x`, so assume they are "blank"
diff --git a/test/gfx/reverse_base_wrap.2bpp b/test/gfx/reverse_base_wrap.2bpp
new file mode 100644
index 00000000..cf3e1ff4
--- /dev/null
+++ b/test/gfx/reverse_base_wrap.2bpp
@@ -0,0 +1 @@
+ÃðÃðÃðÃðÃðÃðÃðÃð<<<<<<<<ðÃðÃðÃðÃðÃðÃðÃðÃ<<<<<<<<ÃÌÃÌÃÌÃÌÃÌÃÌÃÌÃÌ33333333ðÃðÃðÃðÃðÃðÃðÃðÃ<<<<<<<<
\ No newline at end of file
diff --git a/test/gfx/reverse_base_wrap.flags b/test/gfx/reverse_base_wrap.flags
new file mode 100644
index 00000000..431b5aaf
--- /dev/null
+++ b/test/gfx/reverse_base_wrap.flags
@@ -0,0 +1,5 @@
+-b 255
+-l 3
+-t reverse_base_wrap.tilemap
+-q reverse_base_wrap.palmap
+-p reverse_base_wrap.pal
diff --git a/test/gfx/reverse_base_wrap.pal b/test/gfx/reverse_base_wrap.pal
new file mode 100644
index 0000000000000000000000000000000000000000..17adc089d9ab0ecd73ab4573a865bfcff13b4eb9
GIT binary patch
literal 64
zcmb<tcBqrjk!Pr9ejvP`QJ&#JJ%|>o0rBN?z%)?Aq3%Kbeny~52A~?CC`cVhpI8k5
D$siUv

literal 0
HcmV?d00001

diff --git a/test/gfx/reverse_base_wrap.palmap b/test/gfx/reverse_base_wrap.palmap
new file mode 100644
index 00000000..acffa6ca
--- /dev/null
+++ b/test/gfx/reverse_base_wrap.palmap
@@ -0,0 +1,2 @@
+
+	
\ No newline at end of file
diff --git a/test/gfx/reverse_base_wrap.tilemap b/test/gfx/reverse_base_wrap.tilemap
new file mode 100644
index 0000000000000000000000000000000000000000..4c8f6b7620089fdf6e73184bfa46d4b24c97d148
GIT binary patch
literal 8
Pcmey*z{teR!pa5!2si;1

literal 0
HcmV?d00001